To find the most suitable indicator of viral and parasitic contamination of drinking water, large-volume samples were collected and analyzed for the presence of pathogens (cultivable human enteric viruses, Giardialambia cysts, and Cryptospodium oocysts) and potential indicators (somatic and male-specific coliphages, Clotridium perfringens).

The samples were obtained from three water treatment plants by using conventional or better treatments (ozonation, biological filtration).

All samples of river water contained the microorganisms sought, and only C. perfringens counts were correlated with human enteric viruses, cysts, or oocysts.